When heating of liquid is continued, the liquid temperature gradually rises and reaches “a saturation temperature” wherein the liquid temperature does not rise any more. If the liquid is further heated, “vaporization of liquid” occurs in the liquid. This state is boiling, and the above-described saturation temperature is called a boiling point.
In the boiling state, the liquid temperature does not rise, and the energy to be added to liquid by heating is consumed in “vaporizing the liquid inside of the liquid”. This heat energy is called “latent heat”. The latent heat is extremely large as compared with the heat energy raising the temperature of liquid. Accordingly, by using boiling of liquid, a large cooling effect can be obtained.
Cooling using boiling is called “boil cooling”, and heretofore, various boil cooling apparatuses have been proposed.
For example, a boil cooling apparatus of an immersion method has been proposed, which is constituted of a vessel accommodating liquid for cooling and a pipe running through the liquid for cooling in the vessel, and in which boil cooling is performed with a semiconductor element as an object to be cooled immersed in the liquid for cooling, and liquid having a boiling point lower than the liquid for cooling is circulated in the pipe (see, for example, patent document 1).
Boiling phenomenon generally follows the development as described below. The surface of “a heating block” constituted of, for example, metal, etc. is immersed in liquid, and the heating block is heated to raise the heating surface temperature thereof. When the heating surface temperature rises to a certain extent, “a minute bubble about 1 mm or smaller in size” is generated at the surface of the heating block. This state is a state in which the temperature of a liquid layer part of the liquid in contact with the surface of the heating block has reached a saturation temperature, and boiling is occurring at the above-described surface part of the heating block.
As the physical quantity expressing the effect of cooling by boiling of liquid, there is “heat flux”. Expressing in the context of examples in the description, heat flux is “the amount of heat transferred to liquid per unit time from a unit area of the surface of the heating block (the surface in contact with the liquid)”, and the cooling effect is greater as the heat flux is greater.
As a minute bubble starts to be generated at the surface of the heating block, “the growth rate of heat flux” increases, and if heating the heating block is further continued, the quantity of bubbles generated at the surface of the heating block also increases, and the heat flux also continues to increase at a large growth rate, which however is finally saturated.
“The state in which the heat flux has been saturated” this way is in the state that the heating block surface has been covered with “a large bubble”.
That is, if the quantity of minute bubbles generated at a heating surface increases, generated bubbles coalesce to grow to “a large bubble” so large as to be several centimeters in size though depending on the size of the heating surface. A bubble greatly grown in size this way is “a bubble thin in thickness like the squashed one”, and if such a large bubble adheres to the surface of the heating block, at the part of the heating surface the large bubble has adhered to, the heating block is not in direct contact with the liquid, so that boiling is inhibited, and thereby the heat flux is saturated. The heat flux at this time is called “critical heat flux”.
If the heating block is heated even after the heat flux has reached the critical heat flux, at the part of the surface of the heating block the large bubble has adhered to, the heating surface starts to be dried, and with a rapid rise of the heating surface temperature, the heat flux rapidly decreases, so that the cooling effect rapidly deteriorates. If heating further continues, the heating surface is completely dried at the part thereof covered with the large bubble, and this part turns into “the state wherein the part is covered with a thin film of steam”. At this dried part, the heat energy of the heating block is transferred as radiant heat to the liquid, and the heat flux shifts to increase again, however, because the heating surface is not in contact with the liquid, the temperature of the heating block also increases, and if this temperature exceeds a melting point of the heating block, the heating surface “burns out”.
The boiling form from the state that a minute bubble starts to be generated at the surface of the heating block until when the heat flux reaches the critical heat flux is called “nucleate boiling”, the boiling form from the state that the heat flux is the critical heat flux until when the heat flux decreases and the heat flux shifts to increase again is called “transition boiling”, and the boiling form after the change in the heat flux has shifted to increase again is called “film boiling”, respectively.
That is, if heating the heating block immersed in liquid is continued, the boiling forms of nucleate boiling, transition boiling, and film boiling sequentially develop, finally leading to burning out of the heating block. Generally, after the state in which the heat flux is the critical heat flux, “the process leading to burning out from the transition boiling through the film boiling” develops extremely rapidly, and control thereof is extremely difficult, so that boil cooling has been heretofore commonly performed in “the area of nucleate boiling in which the heat flux is equal to or below the critical heat flux”.
More specifically, in the conventional boil cooling method, it is typical that for example, the heat flux (about 100° C.) of about 100 W/cm2 can be obtained at most to a heating surface 1-2 cm long.
On the other hand, various attempts have been made to obtain a high heat flux, and for example, a cooling apparatus rapidly dissolving boiling bubbles using a nozzle and aiming for a high cooling efficiency has been proposed (see, for example, patent document 2).
The cooling apparatus in the above-described proposal is the one for cooling mainly a semiconductor device as a heat-generating body, and the apparatus carries out a cooling method performing cooling, using two types of nozzles, by emitting a jet of a low temperature refrigerant medium liquid from the first nozzle toward the heat-generating body and at the same time, emitting a jet of the same low temperature refrigerant medium liquid from the second nozzle toward the jet of the low temperature refrigerant medium, that has been emitted from the first nozzle and turned into a high temperature refrigerant medium liquid in a vapor-liquid two-phase state as a result of occurrence of boiling bubbles by the heat of the heat-generating body, to rapidly cool the refrigerant medium liquid emitted from the first nozzle, thereby causing the boiling bubbles to be condensed and dissolved.
According to this proposal, it is envisioned that the heat flux of about 200 W/cm2 can be obtained in the temperature area of about 120° C., however, it is believed that the proposed method is applicable only to cooling of a short heating surface like a semiconductor device.
At the same time, it has been reported that when performing boil cooling while causing cooling liquid to circulate along the cooling surface of an object to be cooled, if the cooling liquid is supplied to the cooling surface after sub-cooling the cooling liquid in advance to “the temperature lower than the saturation temperature”, in a certain time frame after the start of cooling, without generating “shifting to transition boiling”, the nucleate boiling form is kept and satisfactory boil cooling can be realized up to a considerably high temperature area (see non-patent document 1).
When performing boil cooling while causing cooling liquid to circulate along the cooling surface of an object to be cooled, if the cooling liquid has been sub-cooled, the heat from the cooling surface rapidly raises the temperature of the cooling liquid in contact with the cooling surface up to a saturation temperature, and thereafter, causes boiling. The place where the temperature rises and boiling occurs at this time is “a thin layered part of the cooling liquid in the vicinity of the cooling surface”, and it is believed that the cooling liquid in the sub-cooled state, that is, the cooling liquid lower than the saturation temperature in temperature exists in the area outside of this layered part of the cooling liquid.
“The cooling liquid lower than the saturation temperature in temperature”, which exists outside of the layered area where boiling is occurring, decreases “the temperature of the cooling liquid in the layered area” in the state of boiling. Due to this temperature decrease, “a bubble in the cooling liquid in the state of boiling is condensed and/or collapsed”.
Because heat is further transferred from the heating surface to the liquid, if sub-cooled liquid is used, it is possible to increase the critical heat flux.
If liquid that has been sub-cooled to a great sub-cooling degree is used as the cooling liquid, the temperature at the vapor-liquid boundary face between a coalesced bubble grown in the transition boiling area and the liquid decreases, and the bubble is collapsed by condensing (the phenomenon opposite of the boiling phenomenon) into micro-bubbles, so that it comes to that the liquid is supplied to the heating surface, and thereby cooling can be performed without causing the boiling form to shift from transition boiling to film boiling, and the critical heat flux can be increased more than in ordinary cases. This phenomenon is called “micro-bubble emission boiling”.
However, even when performing boil cooling using sub-cooled cooling liquid, with passing of the time that the cooling liquid flows along the cooling surface, the temperature of the whole part of the cooling liquid gradually increases, and the effect of sub-cooling inevitably decreases, so that there is a limit to “performing boil cooling to a large cooling area without causing transition boiling” in a considerably high temperature area.
Recently, the length of a heating surface of an IC package of a high heat-generating density electronic device used in an inverter for power conversion being normally 10-30 cm, a cooling method capable of cooling such a surface to be cooled long in the heating surface is demanded, and further, a cooling method capable of dealing with a wide fluctuation in the heat load for example as in the case that the heat generation immediately rises by rapid acceleration or abnormal driving in an electric automobile is demanded.
Further, it is expected in future that a cooling method enabling obtaining the heat flux of about 300 W/cm2 or more will be required, however, those requirements cannot be met by the conventional cooling method, and the advent of a boil cooling method capable of dealing with those requirements is desired.
Patent document 1: Japanese patent laid-open publication No. 1986-54654
Patent document 2: Japanese patent Laid-open publication No. 1993-136305
Non-patent document 1: “Sub-cooled Flow Boiling with micro-bubble emission” (Proc. 41st Japan Symposium on Heat Transfer, June 2004, Vol. 1, pp. 19-20)